C# 获得 当年1月1号

            //默认当年1月1号
            ViewBag.time1 = new DateTime(DateTime.Now.Year, 1, 1).ToString("yyyy-MM-dd");
            ViewBag.time2 = DateTime.Now.ToString("yyyy-MM-dd");

 new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1).ToString("yyyy-MM-dd");  //获取当月第一天

            string t = new DateTime(DateTime.Now.Year, 1, 1).ToString("yyyy-MM-dd");  //2019-01-01
            string t1 = new DateTime(DateTime.Now.Year, 1, 1).ToString("yyyy-MM-dd HH:mm:ss");  //2019-01-01 00:00:00

      var  开始时间 = new DateTime(DateTime.Now.Year, 1, 1).ToString("yyyy-MM-dd");  //2019-01-01
                var 结束时间 = new DateTime(DateTime.Now.Year, 12, 1).AddMonths(1).AddDays(-1).ToString("yyyy-MM-dd"); //2019-12-31

获取当天  时间

var time = Request.Form["time"].ToString().Trim();
            DateTime min_time;   // 比如时间字符串是 "2020-02-19" 转化为时间格式是 2020-02-19 00:00:00
　　

var  min_time = Convert.ToDateTime(time);

//上传的时间的当天最大时间是 var max_dtime = Convert.ToDateTime(min_time + "23:59:59");

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            DateTime min_time111 = DateTime.Today;

            DateTime max_time111 = DateTime.Today.AddDays(1).AddSeconds(-1);